In general, in tires of the heavy vehicle tire type, the carcass reinforcement is anchored on both sides in the bead region and is surmounted radially by a crown reinforcement consisting of at least two superposed layers formed of threads or cables that are parallel within each layer and crossed from one layer to the next making angles of between 10° and 45° with the circumferential direction. The said working layers that form the working reinforcement may further be covered by at least one layer known as a protective layer made up of reinforcing elements that are advantageously metal and extensible, known as elastic elements. It may also comprise a layer of metal threads or cables of low extensibility which make an angle of between 45° and 90° with the circumferential direction, this ply, known as the triangulation ply, being situated radially between the carcass reinforcement and the first crown ply known as the working ply, formed of parallel threads or cables at angles of 45° at most in terms of absolute value. The triangulation ply forms, with at least the said working ply, a triangulated reinforcement which, under the various stresses to which it is subjected, deforms little, the essential role of the triangulation ply being to absorb the transverse compression loads which is the object of all the reinforcing elements in the crown region of the tire.
In the case of tires for “heavy” vehicles, one single protective layer is usually present and its protective elements are, in most cases, directed in the same direction and at the same angle in terms of absolute value as those of the reinforcing elements of the radially outermost, and therefore radially adjacent, working layer. In the case of construction plant tires intended to run over fairly uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially inner protective layer being crossed with the inextensible reinforcing elements of the radially outer working layer adjacent to the said radially inner protective layer.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it turns during normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane or equatorial plane is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
Certain present-day tires known as “road” tires are intended to run at high speeds and over increasingly long journeys, because of the improvements to the road network and the growth of the motorway network worldwide. All of the conditions under which such a tire is required to run undoubtedly allow an increase in the amount of distance travelled, tire wear being lower, but on the other hand tire endurance is penalized. In order to allow one or even two re-treadings of such tires in order to lengthen their life, it is necessary to preserve a structure and, notably, a carcass reinforcement, the endurance properties of which are sufficient to withstand the said re-treading operations.
The prolonged running of tires constructed in this way, under particularly arduous conditions, effectively causes these tires to reach their limits in terms of endurance.
The elements in the carcass reinforcement are notably subjected to bending and compression stresses during running and these stresses are detrimental to their endurance. The cables that make up the reinforcing elements of the carcass layers are in fact subjected to significant stresses when the tires are running, notably to repeated bendings or variations in curvature which, at the threads, give rise to frictional rubbing and therefore wear, as well as fatigue; this phenomenon is termed “fatigue-fretting”.
In order to perform their function of reinforcing the carcass reinforcement of the tire, the said cables have first of all to have good flexibility and high endurance in bending, and this notably entails their threads having a relatively small diameter, preferably less than 0.28 mm, more preferably less than 0.25 mm, and generally smaller than that of the threads used in the conventional cables for tire crown reinforcements.
The cables in the carcass reinforcement are also subject to phenomena known as “fatigue-corrosion” which are due to the very nature of the cables that encourage the passage of, or which even drain, corrosive agents such as oxygen and moisture. Specifically, the air or water that enters the tire, for example upon damage caused by a cut or more simply as a result of the permeability, albeit low, of the interior surface of the tire, may be conducted along the channels formed within the cables as a result of their structure itself.
All these fatigue phenomena which are generally grouped together under the generic heading of “fatigue-fretting-corrosion” cause progressive degeneration of the mechanical properties of the cables and may, for the most arduous running conditions, have an adverse effect on the life of these cables.
In order to improve the endurance of these cables of the carcass reinforcement, it is notably known practice to increase the thickness of the layer of rubber that forms the internal wall of the cavity of the tire in order best to limit the permeability of the said layer. This layer is usually in part made up of butyl to increase the air tightness of the tire. This type of material has the disadvantage of increasing the cost of the tire.
It is also known practice to modify the construction of the said cables in order notably to increase their penetrability by the rubber and thus limit, or even eliminate, the passage of oxidizing agents along the channels formed within the cables. Tires produced in this way have demonstrated problems with air pockets appearing during the manufacture of the tire.
This is because the various stages of manufacture lead to the formation of occluded air pockets. In the case of tires comprising a carcass reinforcement formed of cables the structure of which forms channels capable of conducting air, these air pockets disappear because the air diffuses into the materials and notably through the said channels that exist within the cables. In the case of tires comprising a carcass reinforcement formed of cables the structure of which is highly penetrated by the rubber, these air pockets remain at the end of the manufacturing steps. All that happens is that these air pockets move during the step of curing the tire, these pockets moving towards regions where a low pressure is applied. The movement of the air is along the carcass reinforcement along passages that exist between the reinforcing elements, the layers of rubber compound covering the reinforcing elements forming caved-in regions parallel to the reinforcing elements prior to the stage of curing the tire. These caved-in regions thus allow the air to move slightly according to the pressure applied to the regions at which the air pockets are located. The pressure or the variations in pressure occur notably during the step of curing the tire or alternatively during the shaping step if there is one.
Depending on their location, the appearance of these air pockets is usually unacceptable and may require the tires to be scrapped, as these air pockets are capable of becoming regions of weakness of the tire. The costs of manufacture then become unacceptable simply as a result of the poor production rates.